Transition metal complexes, manufacturing method thereof, photovoltaic cells and manufacturing method thereof

ABSTRACT

This invention provides a transition metal complex of formula MXY 2 Z and a manufacturing method thereof, wherein M is selected from iron, ruthenium, and osmium; X represents a ligand shown in formula (II) 
     
       
         
         
             
             
         
       
     
     wherein R 1  and R 1 ′ are independently selected from COOH, PO 3 H 2 , PO 4 H 2 , SO 3 H 2 , SO 4 H 2 , and derivatives thereof; Y is selected from H 2 O, Cl, Br, CN, NCO, NCS, and NCSe; Z represents a bidentate ligand having at least two fluorinated chains. In addition, this invention also provides photovoltaic cells and a manufacturing method thereof.

FIELD OF THE INVENTION

This invention relates to transition metal complexes and moreparticularly to transition metal complexes used in the preparation ofdye-sensitized solar cells (DSSCs).

BACKGROUND OF THE INVENTION

Sensitizers are one of the most crucial components for the preparationof DSSCs because they affect not only the incident photon conversionefficiency (IPCE) of the cells but also the stability of the components.

Thus far, various sensitizers have been proposed by scientists in hopesof increasing the efficiency of DSSCs and prolonging their service life.For example, Michael Grätzel, a Swiss scientist, developed in 1999 asensitizer named N719, which was widely used in the industry for itshigh IPCE.

However, because N719 cannot sustain high temperature and falls offeasily after being used for a period of time, solar cells containing thesame usually can no longer work normally after three years of usage.

In order to improve the stability of N719 under the existence of a heatsource or in a moist condition, Grätzel further proposed in 2003 anothersensitizer named Z907. Proved by experiments, Z907 can still possess 94%of its original efficiency after being operated continuously for 1,000hours under 80° C. In contrast, the efficiency of N719 decreases 35%under the same condition.

Although Z907 demonstrates great sustainability in long-term stabilitytesting, Z907 is not completely satisfactory because it has a molarabsorption coefficient lower than N719.

Accordingly, it is highly desirable for scientists to develop novelsensitizers with high IPCE and thermal stability.

SUMMARY OF THE INVENTION

In view of the demands for developing a new generation of sensitizers inthe industry, one of the objectives of this invention is to providenovel transition metal complexes and manufacturing methods thereof,wherein the transition metal complexes are applicable to the preparationof photovoltaic cells.

It is another objective of this invention to provide a transition metalcomplex having a general formula MXY₂Z, wherein M is selected from iron,ruthenium, and osmium; X is a ligand shown by formula (II):

wherein R₁ and R₁′ are independently selected from COOH, PO₃H₂, PO₄H₂,SO₃H₂, SO₄H₂ and derivatives thereof; Y is selected from H₂O, Cl, Br,CN, NCO, NCS and NCSe; and Z is a bidentate ligand with two or morefluorinated chains.

Preferably, M is ruthenium; R₁ and R₁′ are independently selected fromCOOH, PO₃H₂, and derivatives thereof; Y is NCS; Z is bipyridine with atleast two fluorinated chains which are located on different pyridylrings.

Preferably, bipyridine is substituted by a fluorinated functional grouphaving a spacer, which is preferably an ether linkage or at least onemethylene structure. In addition, Z preferably comprises at least onefluorinated chain of formula (IV):

—(CH₂)_(m)—O—(CH₂)_(n)—R_(f)  (IV),

wherein m and n are each independently an integer greater than zero,such as 1, 2 or 3, and R_(f) is a fluorinated alkyl chain, such as—CF₂—CF₂H, —CF₂—CF₂—CF₃, —CF₂—CF₂—CF₂—CF₂H, etc.

Preferably, the fluorinated chains of Z are independently substituted by1 to 30 fluorine atoms and more preferably by 1 to 20 fluorine atoms.Most preferably, the fluorinated chains of Z are independentlysubstituted by 4, 7, 8, 12, 13 or 19 fluorine atoms.

It is another objective of this invention to provide a transition metalcomplex having the following chemical structure:

wherein FC₁ and FC₂ are each independently a fluorinated chain with aspacer.

Preferably, FC₁ and FC₂ can be substituted at position number 4 or 5 ofthe pyridyl ring, and FC₁ or FC₂ can be fluorinated functional grouphaving a spacer, such as —CH₂—O—CH₂—R_(f), wherein R_(f) is a linear orbranched fluorinated alkyl chain, such as an alkyl chain containing 4,7, 8, 12, 13 or 19 fluorine atoms.

It is another objective of this invention to provide a method ofpreparing the above-mentioned transition metal complexes, wherein theimprovement comprises using a chelating agent of the following formula:

wherein R_(f) is an alkyl chain substituted by 1 to 30 fluorine atoms.

It is still another objective of this invention to provide a method ofpreparing photovoltaic cells, the method comprising:

providing a transition metal complex having a general formula MXY₂Z,wherein M is selected from iron, ruthenium, and osmium; X is a ligandshown by formula (II):

wherein R₁ and R₁′ are independently selected from COOH, PO₃H₂, PO₄H₂,SO₃H₂, SO₄H₂ and derivatives thereof; Y is selected from H₂O, Cl, Br,CN, NCO, NCS and NCSe; and Z is a bidentate ligand with two or morefluorinated chains; and

preparing photovoltaic cells by using the transition metal complex.

It is yet another objective of this invention to provide a photovoltaiccell, which comprises an anode having a conductive substrate and ametal-oxide-semiconductor layer formed on the conductive substrate, themetal-oxide-semiconductor layer being treated by a sensitizing dye; acounter electrode; and an electrolyte provided between the counterelectrode and the metal-oxide-semiconductor layer, wherein thesensitizing dye is a bidentate ligand with at least two fluorinatedchains.

It is yet another objective of this invention to provide a transitionmetal complex and a method of manufacturing the same. The transitionmetal complex has unexpectedly high IPCE and desirable thermal stabilityand thereby is applicable to the production of photovoltaic cells withgreat quality.

BRIEF DESCRIPTION OF THE DRAWINGS

The structure and the technical means adopted by the present inventionto achieve the above and other objects can be best understood byreferring to the following detailed description of the preferredembodiments and the accompanying drawings, wherein

FIG. 1 shows the main steps for preparing a transition metal complex ofthis invention;

FIG. 2 is an illustrative diagram of a DSSC containing a transitionmetal complex of this invention;

FIG. 3 is the plot of IPCE versus wavelength of a DSSC containing atransition metal complex of this invention; and

FIG. 4 is the plot of IPCE versus wavelength of a DSSC containinganother transition metal complex of this invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Example 1 Preparationof Transition Metal Complexes

FIG. 1 shows the main steps for preparing a transition metal complex ofthis invention, from which it can be observed that bipyridine containing—(CH₂)_(m)—O—(CH₂)_(n)—R_(f) is used to chelate the transition metal.Details for the preparation of the chelating agent can be found at N.Lu, J-Y Chen, C-W Fan, Y-C Lin, Y-S Wen, L-K Liu, J. Chin. Chem. Soc,2006, 53, 1517-1521; N. Lu, Y-C Lin, J-Y Chen, C-W Fan, L-K Liu,Tetrahedron, 2007, 63, 2019-2023; N. Lu, Y-C Lin, J-Y Chen, T-C Chen,S-C Chen, Y-S Wen, L-K Liu, Polyhedron. 2007, 26, 3045-3053; and N. Lu,Y-C Lin, Tetrahedron Lett. 2007, 48, 8823-8828, all of which areincorporated by reference herein. In the above-mentioned formula, m andn are each independently an integer greater than zero, and when R_(f) is—CF₂—CF₂H, —CF₂—CF₂—CF₃, and —CF₂—CF₂—CF₂—CF₂H, the transition metalcomplexes produced thereby are named CT4, CT7 and CT8, respectively.

1-1: Preparation of CT8

Dichloro(p-cymene)-ruthenium(II) dimer (Aldrich, 0.38 g, 0.62 mmol) andbipyridine having substitution of eight fluorine atoms at each ring (0.8g, 1.24 mmol) were dissolved in 60 ml ethanol and then the solution wasstirred and refluxed for 8 hours at 80° C. under N₂ atmosphere. Afterpumping away ethanol, Bpy-COOH (0.30 g, 1.24 mmol) and 40 mL dry DMFwere added. The reaction mixture was refluxed at 140° C. for another 4hours at dark. Excess NH₄NCS (SHOWA, 2.92 g, 38.44 mmol) was added tothe reaction mixture and heated at 130° C. for 5 hours. After reaction,the solvent was removed with a rotary vacuum pump and large amount ofwater was added to dissolve the excess NH₄NCS. Then 1.18 g (1.06 mmol)dark purplish red solid product CT8 was obtained after vacuumfiltration.

In order to obtain purer dye, the solid product was treated with TBAOH(tetrabutylammonium hydroxide), and the resulting TBA salt was dissolvedin methanol then passed through the chromatography column (SephadexLH20) using methanol as an eluent. The main band was collected andconcentrated, and the solvent was extracted by an evaporator. Theprocess was repeated five times; then some water was added, and 0.02 MHNO₃ was added to adjust the pH value to precipitate the product. Theproduct was placed in a refrigerator for 24 hours, followed by vacuumfiltration at room temperature to obtain CT8-TBA.

1-2 Identification Data of CT8 and CT8-TBA

Identification Data of CT8

H-NMR (500 MHz, CD₃OD), δ (ppm):

9.61 (d, H₆, ³J_(HH)=5.5 Hz); 9.36 (d, H₆′, ³J_(HH)=5.5 Hz); 9.01 (s,H₃); 8.86 (s, H₃′); 8.49 (s, H₃′″); 8.34 (s, H₃″); 8.21 (d, H₆′″,³J_(HH)=6.4 Hz); 7.84 (d, H₅, ³J_(HH)=5.5 Hz); 7.79 (d, H₆″, ³J_(HH)=4.6Hz); 7.60 (d, H₅′, ³J_(HH)=5.5 Hz 7.52 (d, H₅′″, ³J_(HH)=6.4 Hz); 7.15(d, H₅″, ³J_(HH)=4.6 Hz); 6.59 (tt, H₁₀′″, ²J_(HF)=50.8 Hz, J³ _(HF)=5.5Hz, 2H); 6.50 (tt, H₁₀″, ²J_(HF)=50.8 Hz, ³J_(HF)=5.5 Hz, 2H); 5.06 (s,H₈′″, 2H); 4.79 (s, H₈″, 2H); 4.37 (t, H₉′″, ³J_(HF)=14 Hz, 2H); 4.17(t, H₉″, ³J_(HF)=14 Hz, 2H)

¹³C-NMR (113 MHz, CD₃OD), δ (ppm):

166.9 (C₇); 166.6 (C₇′); 160.9 (C₂); 159.7 (C₂′); 159.6 (C₂″); 158.3(C₂′″); 155.1 (C₆); 154.0 (C₆′); 153.8 (C₆″); 152.9 (C₆′″); 149.9 (C₄);149.2 (C₄′); 139.9 (C₄″); 139.3 (C₄′″); 127.0 (C₃); 126.3 (C₃′); 125.8(C₃″); 125.1 (C₃′″); 123.7 (C₅); 123.6 (C₅″); 122.0 (C₅″); 122.0 (C₅′″);135.3 (C₁₄ of NCS); 134.5 (C₁₄′ of NCS); 106-118 (C₁₀′″˜C₁₃′″ andC₁₀″˜C₁₃′″); 73.2 (C₈′″); 72.8 (C₈″); 69.0 (C₈″); 68.7 (C₉″)

¹⁹F-NMR (470.5 MHz, CD₃OD), δ (ppm):

−121.3 (t, —CH₂CF₂CF₂—, ³J_(HF)=12.7 Hz); −121.4 (t, —CH₂CF₂CF₂—,³J_(HF)=13.5 Hz); −126.7 (s, —CH₂CF₂CF₂—); −126.9 (s, —CH₂CF₂CF₂—);−131.9 (d, —CF₂CF₂H); −132.0 (d, —CF₂CF₂H); −140.1 (t, —CF₂H,²J_(HF)=45.2 Hz); −140.1 (t, —CF₂H, ²J_(HF)=40.1 Hz)

FT-IR υ (cm⁻¹):

2105 (N═C stretch, s); 1718 (C═O stretch, s); 1617, 1543, 1406(bipyridine ring, m); 1613 (—COO⁻ stretch, as), 1383 (—COO⁻ stretch, s);1258, 1230 (—C—O stretch, s); 1169 (CF₂ stretch, as); 1126 (CF₂ stretch,s)

HR-FAB M⁺:

C₃₆H₂₄F₁₆N₆O₆RuS₂, Calcd m/z 1105.9969, found m/z 1105.9962 (accurate to3 decimal places)

Identification Data of CT8-TBA

¹H-NMR (500 MHz, CD₃OD), δ (ppm):

9.47 (d₆, ³J_(HH)=5.5 Hz, 1H); 9.42 (d₆′, ³J_(HH)=6.0 Hz, 1H); 8.98 (s,1H); 8.82 (s, 1H); 8.98 (s, 1H); 8.33 (s, 1H); 8.18 (d, ³J_(HH)=5.8 Hz,1H); 7.79 (d, ³J_(HH)=6.0 Hz, 1H); 5.5 Hz, 2H); 7.53 (d, ³J_(HH)=6.0 Hz,1H); 7.65 (d, ³J_(HH)=7.52 (d, ³J_(HH)=6.0 Hz, 1H); 7.15 (d, ³J_(HH)=5.5Hz, 1H); 6.66 (tt, H₁₀′″, ²J_(HF)=51.5 Hz, J³ _(HF)=5.5 Hz, 2H); 6.97(tt, H₁₀″, ²J_(HF)=51.5 Hz, J³ _(HF)=5.5 Hz, 2H); 5.07 (s, H₈′″, 2H);4.80 (s, H₈″, 2H); 4.36 (t, H₉′″, ³J_(HF)=14 Hz, 2H); 9.17 (t, H₉″,³J_(HF)=14 Hz, 2H); 1.64 (m, 2H); 1.38 (m, 2H); 0.99 (t, ³J_(HH)=7.5,3H)

¹³C-NMR (113 MHz, CD₃OD) δ (ppm):

170.7 (C₇); 170.4 (C₇′); 160.5 (C₂′); 160.0 (C₂′); 159.2 (C₂″); 158.8(C₂′″); 154.3 (C₆); 154.1 (C₆′); 152.8 (C₆″); 152.7 (C₆′″); 149.2 (C₄);148.6 (C₄′); 147.5 (C₄″); 146.8 (C₄′″); 127.0 (C₃); 126.2 (C₃′); 125.6(C₃″); 125.1 (C₃′″); 123.6 (C₅); 123.4 (C₅′); 121.9 (C₅′″); 121.8(C₅′″); 134.3 (C₁₄ of NCS); 134.2 (C₁₄′ of NCS); 106-118 (C₁₀′″˜C₁₃′″and C₁₀″˜C₁₃″); 73.2 (C₈′″); 72.9 (C₈″); 68.9 (C₉′″); 68.7 (C₉″); 59.5(—CH₂CH₂CH₂CH₃); 24.8 (—CH₂CH₂CH₂CH₃); 20.7 (—CH₂CH₂CH₂CH₃); 14.0(—CH₂CH₂CH₂CH₃);

¹⁹F-NMR (470.5 MHz, CD₃OD), δ (ppm):

−121.3 (t, —CH₂CF₂CF₂—, ³J_(HF)=13.1Hz); −121.5 (t, —CH₂CF₂CF₂—,³J_(HF)=10.8 Hz); −126.8 (s, —CH₂CF₂CF₂—); −126.9 (s, —CH₂CF₂CF₂—);−132.0 (d, —CF₂CF₂H); −132.1 (d, —CF₂CF₂H); −140.1 (t, —CF₂H,²J_(HF)=45.2 Hz); −140.1 (t, —CF₂H, ²J_(HF)=45.1 Hz)

FT-IR υ (cm⁻¹):

2105 (N═C stretch, s); 1618, 1543, 1420 (bipyridine ring, m); 1610(—COO⁻ stretch, as); 1383 (—COO⁻ stretch, s); 1259, 1229 (—C—O stretch,s); 1169 (CF₂ stretch, as); 1127 (CF₂ stretch, s)

It should be noted that the transition metal complexes of this inventionmay comprise a fluorinated chain substituted by different numbers offluorine atoms, such as 4, 7, 8, 12, 13, or 19, and the synthesis methodthereof is similar to FIG. 1, except that different fluorinated chainsare used in the chelating agent bipyridine ring. Accordingly, ifdifferent chelating agents are used, a person skilled in the art cansynthesize the transition metal complexes with a different fluorinatedchain without undue experimentation. Moreover, the substitution positionof the fluorinated chain on the pyridyl ring is not limited to positionnumber 4; a fluorinated chain of other substitution positions can alsobe synthesized without undue experimentation by using a similar method.

Example 2 Preparation of DSSCs

In order to measure various data of the transition metal complexes ofthis invention applied to DSSCs, TiO₂ thin film electrode with an activearea controlled at a dimension of 0.25 cm² with a thickness of 16 μm wasprovided, heated to 80° C. and dipped into the THF solution containing3×10⁻⁴ M dye sensitizers for 24 hours. The counter electrode was FTOconductive glass coated with Pt electrode, and the electrolyte wascomposed of 0.5 M lithium iodide (LiI), 0.05 M iodine (I₂), and 0.5 M4-tert-butylpyridine dissolved in acetonitrile. The electrolyte wasinjected onto the surface of the counter electrode, and the TiO₂electrode and the counter electrode were tightly sealed to prevent thegeneration of bubbles. Then a foldback clip was used to fasten theelectrodes, such that a DSSC with a sandwich-like structure shown inFIG. 2 was obtained, in which conductive glass is represented by numeral1, dye-containing TiO₂ by 2, electrolyte by 3, Pt layer by 4 and theother conductive glass by 5.

Example 3 Measurement of Dye Performance

The performance of the dyes after incorporated into a solar cell isshown below:

Dyes A, B and C are incorporated into three solar cells respectively, inwhich the maximal conversions are obtained when the wavelength of theincident light is at 540 nm. The maximum IPCE measured are A (67.7%), B(70.4%), C (70.2%) and N719 (69.5%). It can be observed that Dye B hasthe highest conversion efficiency greater than Dye C and Dye A, whichhas the lowest conversion efficiency. In addition, Dye B has an IPCEgreater than that of N719 within wavelength 360 nm˜540 nm, and Dye C hasan IPCE greater than that of N719 within wavelength 440 nm˜600 nm, asshown in FIG. 3. Although Dye A does not have an IPCE greater than N719,its IPCE reaches up to 97.4% of N719. Therefore, the IPCEs of Dyes A, Band C reflect the high performance of the dyes overall.

Detailed photovoltaic parameters under AM1.5 of cells comprising Dyes A,B, C and N719 are shown in Table 1:

TABLE 1 J_(sc) Dye V_(oc) (V) (mA/cm⁻²) FF η (%) A 0.67 13.33 0.70 6.25B 0.68 15.44 0.66 6.93 C 0.68 14.98 0.67 6.82 N719 0.71 15.37 0.67 7.31Z907 0.68 14.16 0.66 6.36

wherein Voc represents Open Circuit Voltage; J_(SC) represents ShortCircuit Current; FF represents Fill Factor; η represents the overallefficiency of the cell.

In addition, detailed photovoltaic parameters under AM1.5 of cellscomprising Dyes D, E, F and N719 are shown in Table 2:

TABLE 2 Jsc Dye Voc (V) (mA/cm²) FF η (%) D 0.65 9.59 0.72 4.48 E 0.6913.88 0.66 6.32 F 0.66 10.94 0.72 5.11 N719 0.72 15.24 0.64 7.02

Example 4 Measurement of Dye Performance (II)

FIG. 4 is the plot of IPCE versus wavelength of DSSCs containingCT9-TBA, CT7-TBA, and CT8-TBA respectively, and the detailedphotovoltaic parameters under AM1.5 of the cells are shown in Table 3:

TABLE 3 J_(sc) Dye V_(oc) (V) (mA/cm²) FF η (%) CT4-TBA 0.65 13.48 0.675.87 CT7-TBA 0.71 13.80 0.66 6.46 CT8-TBA 0.71 13.69 0.66 6.41 N719 0.7115.37 0.67 7.31

Example 5 Stability Test of Dyes

I. Dye Adsorption

TiO₂ thin film electrodes (14 μm in thickness and 3 cm² in dimension)coated on FTO conductive glass by the sol-gel process were disposed intoa 100° C. oven for 3 hours to remove water. Then 15 mL dyes (includingDye B, Dye C and N719 dissolved in DMF, 2×10⁻⁴ M) were prepared, and 5mL of each was used as the reference for spectrum scanning by a UV/Visspectrophotometer (1 cm path length) to obtain the absorption of theeach dye. The exact concentration of each dye before the electrodes weresoaked was calculated by using the Beer-Lambert law (formula 1-1) withthe molar extinction coefficient of each dye. Moreover, the electrodefilms coated on the conductive glasses were soaked in the residual 10 mLof each dye, which was used as the working sample, for 12 hours. Afteradsorption balance was reached, the electrodes were taken from the dyes,and DMF was used to wash the dyes/TiO₂ thin films to break themulti-layered bonding of physical adsorption on the thin films.Similarly, the UV/Vis spectrophotometer was used to measure theadsorption of the working samples, and the Beer-Lambert law was used tocalculate the exact concentration of each dye after the electrodes weresoaked. The amounts of dyes adsorbed by the electrode thin films wereobtained by subtracting the number of mole of each dye after theelectrodes were soaked from the number of mole of each dye before theelectrode was soaked. Then the dyes/electrode thin films coated on theconductive glasses were scraped off to measure their weights, and theadsorption amount of each dye on the respective TiO₂ thin film electrodewere calculated by formula 1-2:

A=εBC  (1-1),

-   -   wherein A: absorption; ε: molar extinction coefficient; B: path        length; C: concentration of sample

Dye adsorption amount=[amount of dye adsorbed by TiO₂ electrode/totalamount of adsorbed dye and TiO₂ electrode]×2.4  (1-2)

The adsorption result is shown below:

TABLE 4 Dye B C N719 Amount of dye before TiO₂ 3 3 3 electrode wassoaked (×10⁻⁶ mol) Amount of dye after TiO₂ 2.622 2.623 2.740 electrodewas soaked (×10⁻⁶ mol) Amount of dye adsorbed by TiO₂ 0.378 0.377 0.26electrode (×10⁻⁶ mol) Total amount of adsorbed dye and 3.6 3.7 3.3 TiO₂electrode (mg) Dye adsorption calculated by 2.52 2.45 1.89 formula 1-2(×10⁻⁷ mol/cm²)

From Table 4, it can be observed that, compared with N719, which iswidely known for its high performance, Dye B and Dye C have a greateradsorption amount; thus, the dyes of this invention can provide anenhanced performance.

II. Dye Desorption

Dyes/TiO₂ electrode thin films (including 3×10⁻⁴ M Dye B, Dye C, andN719; the electrode thin films have a dimension of 0.25 cm² with athickness of 16 μm) soaked in THF for 12 hours to reach adsorptionbalance were treated by alkali (5M NaOH solution) to completely wash offthe dye adsorbed on the surface of the thin film. The desorption resultis as follows:

Dye B C N719 Z907 Desorption? No No Yes Yes

As shown above, Dye B and Dye C both demonstrate long-term stabilitybetter than N719 and have stronger resistance to strong alkali, so theydo not fall off easily after being used for a period of time.Particularly, even when compared with Z907, which is widely known forits long-term stability, the dyes of this invention demonstratesstability better than Z907. Therefore, solar cells using the dyes ofthis invention have a longer service life.

The present invention has been described with a preferred embodimentthereof and it is understood that many changes and modifications in thedescribed embodiment can be carried out without departing from the scopeand the spirit of the invention that is intended to be limited only bythe appended claims.

1. A transition metal complex of formula (I):MXY₂Z  (I) wherein M is selected from iron, ruthenium, and osmium; X isa ligand shown by formula (II):

wherein R₁ and R₁′ are independently selected from COOH, PO₃H₂, PO₄H₂,SO₃H₂, SO₄H₂ and derivatives thereof; Y is selected from H₂O, Cl, Br,CN, NCO, NCS and NCSe; and Z is a bidentate ligand with two or morefluorinated chains.
 2. The transition metal complex according to claim1, wherein Z is a ligand shown by formula (III):

wherein R₂ and R₂′ are each independently a fluorinated functional grouphaving a spacer.
 3. The transition metal complex according to claim 2,wherein the spacer has an ether linkage structure.
 4. The transitionmetal complex according to claim 2, wherein the spacer has a —CH₂—O—CH₂—structure.
 5. The transition metal complex according to claim 1, whereinZ comprises at least one fluorinated chain with a spacer.
 6. Thetransition metal complex according to claim 1, wherein Z comprises atleast one fluorinated chain of formula (IV):—(CH₂)_(m)—O—(CH₂)_(n)R_(f)  (IV), wherein m and n are eachindependently an integer greater than zero, and R_(f) is a fluorinatedalkyl chain.
 7. The transition metal complex according to claim 1,wherein the fluorinated chains of Z are independently substituted by 1to 30 fluorine atoms.
 8. The transition metal complex according to claim1, wherein the fluorinated chains of Z are independently substituted by4, 7, 8, 12, 13 or 19 fluorine atoms.
 9. The transition metal complexaccording to claim 1, having the following chemical structure:

wherein FC₁ and FC₂ are each independently a fluorinated chain with aspacer.
 10. The transition metal complex according to claim 9, whereinFC₁ and FC₂ are each independently —CH₂—O—CH₂—R_(f), and R_(f) is afluorinated alkyl chain.
 11. The transition metal complex according toclaim 9, wherein R_(f) is an alkyl chain substituted by 1 to 30 fluorineatoms.
 12. The transition metal complex according to claim 11, whereinR_(f) is an alkyl chain substituted by 1 to 20 fluorine atoms.
 13. Thetransition metal complex according to claim 9, wherein M is ruthenium.14. A method of preparing photovoltaic cells, comprising: providing atransition metal complex of claim 1; and preparing photovoltaic cellscontaining the transition metal complex.
 15. The method according toclaim 14, wherein the transition metal complex has the followingchemical structure:

wherein FC₁ and FC₂ are each independently a fluorinated chain with aspacer.
 16. The method according to claim 15, wherein FC₁ and FC₂ areeach independently —CH₂—O—CH₂R_(f), and R_(f) is an alkyl chainsubstituted by 1 to 30 fluorine atoms.
 17. A method of preparing atransition metal complex of claim 9, wherein the improvement comprisesusing a chelating agent of the following formula:

wherein R_(f) is an alkyl chain substituted by 1 to 30 fluorine atoms.18. A photovoltaic cell, comprising: an anode having a conductivesubstrate and a metal-oxide-semiconductor layer formed on the conductivesubstrate, the metal-oxide-semiconductor layer being treated by asensitizing dye; a counter electrode; and an electrolyte providedbetween the counter electrode and the metal-oxide-semiconductor layer,wherein the sensitizing dye is a transition metal complex of claim 1.